"be silent," 1560s, from Middle English mum, mom (late 14c.), inarticulate closed-mouth sound, indicative of unwillingness or inability to speak. As an adjective meaning "secret" from 1520s. Phrase mum's the word is first recorded 1704.

pet word for "mother," 1823, short for mummy (see mamma). In British sociology, used from 1957 in reference to "the working class mother as an influence in the lives of her children." Also sometimes a vulgar corruption of madam.